Generally, gear mechanisms such as a trapezoidal thread worm gear mechanism or a rack and pinion gear mechanism are used as a mechanism to convert rotary motion of an electric motor to axial linear motion in an electric linear actuator in various types of driving sections. These motion converting mechanisms involve sliding contact portions. Thus, power loss is increased. Accordingly, the size of electric motor and power consumption are increased. Thus, ball screw mechanisms have been widely adopted as more efficient actuators.
In prior art electric linear actuators, an output member, connected to a nut, can be axially displaced by rotationally driving a ball screw shaft, forming a ball screw, with the use of an electric motor supported on a housing. Usually, friction of the ball screw mechanism is very low. Thus, the ball screw shaft tends to be easily reversely rotated by a pushing thrust load applied to the output member. Accordingly, it is necessary to hold the position of the output member when the electric motor is stopped.
An electric linear actuator has been developed with a brake for the electric motor or a low efficient mechanism, such as a worm gear is provided as a power transmitting mechanism. One representative example of the electric linear actuator is shown in FIG. 11. Here, the electric linear actuator 50 has a cylindrical housing 51 with a cavity 51a to contain a ball screw mechanism. A cylinder portion 51b is arranged coaxially with the cavity 51a. A fluid inlet (not shown) and fluid outlet 51c communicate with the cylinder portion 51b. 
A screw shaft 52 extends into the cavity 51a of the housing 51. One end of the screw shaft 52 is connected to an electric motor (not shown) arranged outside of the housing 51. The outer circumference of the screw shaft 52 includes a male-screw groove 52a, a round shaft portion 52b and flange portion 52c, therebetween. An inner ring 53a of a bearing 53 is fit on the round shaft portion 52b. The inner end face (the right end face in FIG. 11) of the inner ring 53a abuts against the flange portion 52c. An outer end face (the left end face) of an outer ring 53b of the bearing 53 abuts against a stopper ring 54 fit in the cavity 51a. Accordingly, the screw shaft 52 is rotationally supported by the bearing 53, but is axially immovable. In addition, a washer 55 and a leaf spring (shock absorbing member) 56 are disposed between the inner end face of the outer ring 53b of the bearing 53 and a stepped portion 51d of the housing 51.
A cylindrical nut 57, only axially movable relative to the housing 51, is disposed around the screw shaft 52. It includes a female-screw groove 57a on its inner circumference. A plurality of balls 58 are rollably disposed within a helical rolling passage formed between the male and female screw grooves 52a, 57a. The ball screw mechanism is thus formed by the screw shaft 52, the nut 57 and the balls 58.
A radially projecting portion 57b, with a rectangular cross-section, is integrally formed with the outer circumference of the nut 57. The radially projecting portion 57b engages an axially extending guide groove 51e formed on the inner circumference of the cavity 51a of the housing 51. A predetermined gap “δ” is formed in each space between the side surfaces (engaging surfaces) 57c, 57c and the side surfaces (guide surfaces) 51f, 51f of the guide groove 51e. 
A tube 57d, forming a ball-circulating member, is mounted on a flat outermost surface of the radially projecting portion 57b. The tube 57d is secured on the nut 57 by screws 57f via a bracket 57e. The tube 57d returns the balls 58 from one end to the other end of the helical rolling passage formed between the screw grooves 52a, 57a. 
A hollow cylindrical piston member 59, having one closed end, is mounted on the right end of the nut 57. The screw shaft 52 can enter into and come out from a space within the piston member 59. The outer circumference of the piston member 59 is closely and slidably fit into the inner circumference of the cylinder portion 51b of the housing 51. An O-ring 60 is disposed in a circumferential groove 59a formed near the right end of the piston member 59. The O-ring 60 prevents fluid filled within the cylinder portion 51b from leaking toward the cavity 51a through a gap between the piston member 59 and the cylinder portion 51b (see e.g. JP 2006-233997 A).
In the prior art electric linear actuator 50, the anti-rotation of the nut 57 is performed by engagement of the guide groove 51e, formed on the housing 51, with the radially projecting portion 57b, integrally formed on the nut 57. However, the integral structure of the radially projecting portion 57b with the nut 57 increases manufacturing cost of the electric linear actuator 50. In addition, the housing 51 is formed of aluminum alloy to reduce the weight of the electric actuator 50. However, the housing 51, formed of aluminum alloy, would be short in wear resistance and strength for the engagement of the guide groove 51e of the housing 51 with the radially projected portion 57a. Also, it is believed that the housing 51 would be deformed or destroyed by an impact force of the ball screw applied against the inner wall of the housing 51 if the electric linear actuator 50 could not be correctly controlled due to system error etc.